The use of composites for producing various structures is well known, particularly for aircraft, automotive or space applications. Composites have the advantage of high strength and low weight, reducing the energy requirements of automobiles and aircraft. For satellites or other space structures, composites reduce the lift requirements for placing these structures in orbit.
As presently envisioned, any future space station will include a plurality of pressurized structures integrated by interconnecting trusses, all composed of composite materials. Such large space structures as the pressurized modules will require a highly reliable structural monitoring system because of the potential vulnerability to micrometeor damage. Identification and prompt location of any puncture is required to maintain safety and operational reliability. However, there is no adequate system presently available for monitoring the structures and detecting sites of impact damage to allow rapid location and repair.
One method for damage detection involves closely spaced embedded optical fibers in an X-Y coordinate pattern. A plurality of single straight fiber optic strands are placed in a composite. Once a fiber is broken, light transmission is interrupted and the location of the damage determined. In large structures, however, if each adjacent strand in the fiber matrix is an individual fiber, independent from all of the rest, a substantial number of fibers are required. This, in turn, requires an automatic electronic readout system that is highly complex. In addition, the fiber bundles going in and out of the structure will be bulky and heavy. For example, one of the proposed space station modules has a structural surface area on the order of 2,500 square feet, comparable to a 50.times.50 foot square area. To detect a hole of 1/8" diameter anywhere in such a structure, a pattern of fibers with a 1/8" spacing would be required. An X and Y matrix to precisely locate the hole would require two sets of 4,800 fibers, each fiber 50-foot long and laid at right angles to each other for a total of 9,600 fibers. For automatic readout, an equal number of photo diodes to detect light transmission through the fibers from the source or a method of sequencing light input from the 9,600 fibers would be required. Such a system would be highly complex and prohibitive in terms of cost and weight.
Another application for a damage detection system is in composite aircraft structures subject to high stress where crack detection or impact damage detection is critical to aircraft survival. For such aircraft applications, the damage detection system must be of minimum complexity and weight to prevent a reduction in aircraft performance. Therefore, the straight X-Y fiber grid system would be unsuitable.